A natural history notebook and project of the Missouri Botanical Garden

Mesoamerica

Damaged ecosystems don’t recover overnight, but sometimes that’s all the time that they get. CCSD scientist Leighton Reid describes new research about tropical secondary forests in southern Costa Rica, including how long these young forests last, what’s at stake, and how we can keep them around longer.

Regrowing tropical forests on marginal farm lands is one of the main ways that humans can prevent runaway climate change. With ample moisture and long growing seasons, tropical trees often can grow quickly and pull large amounts of carbon out of the atmosphere, storing it in their wood and keeping it from trapping heat. At the same time, young forests provide habitat for plants and animals and improve water quality for humans, among many other benefits.

But even in a moist, tropical climate, trees don’t grow instantly. Typically, it takes many decades for a recovering forest to stock up all of the carbon that it can hold. And it can take even longer for some plants (like orchids) and animals (like antbirds) to return. If a forest starts to grow back, but then someone cuts it down again, these time-dependent benefits never accrue.

In other words, the hopes and expectations that many people have for young tropical forests depend on young tropical forests growing old. So do they? Our new study suggests not.

To find out how long secondary forests persist, I teamed up with Matthew Fagan, a landscape ecologist at the University of Maryland Baltimore County, and Rakan Zahawi, director of the Lyon Arboretum, as well as two students, James Lucas at Washington University and Joshua Slaughter at UMBC.

We studied a set of historical, aerial photos from southern Costa Rica, which covered the time period from 1947-2014. Previously, Zahawi and colleagues had classified which areas in each photo were forest and which areas were farms or other non-forest land uses. By comparing the maps they made for each year, we were able to see where and when new forests appeared and how long they remained as forest before they were converted to some other land use (mostly farms).

The young forests did not last long. Half of the new forests disappeared before they were 20-years old. And 85% were cut down before they were 54-years old. Larger forests and forests near rivers lasted longer.

First, the bad news. Twenty years is not even close to the amount of time it takes for a young forest to become as diverse as an old-growth forest. For example, vascular epiphytes like orchids and bromeliads take more than 100 years to fully recover in young forests.

Carbon storage will also take a hit. If forests elsewhere in Latin America are as ephemeral as forests in southern Costa Rica, then carbon stocking over the next thirty years may be reduced by an order of magnitude.

Ephemeral forests could just be a problem in Costa Rica, but another study shows that secondary forests in eastern Peru have even shorter lifespans. There, secondary forests are cleared at a rate of 3-23% per year. Compared to that, the 2-3% per year rate of loss in southern Costa Rica is considerably better. And that’s not a good thing. Clearly we need more research on secondary forest persistence from other places.

There is some good news, though. Even though many new forests were short-lived, the ones that survived were predictable. And if we can predict where new forests will survive, we should also be able to help them survive longer. Larger forests and forests close to rivers were cut down less often than small forests and forests far from rivers. This suggests that restoring large, riparian forests could be a smart investment.

Governments and other organizations can also help forests persist by creating incentives for long-term forest management, providing resources to enable long-term management, and ensuring that local people will be able to enjoy the benefits that old forests provide.

We hope that this work will lead to stronger restoration commitments. Right now, dozens of countries are setting big targets for forest restoration. For example, in 2012 Costa Rica committed to restore a million hectares of degraded land by 2020 (an area about one fifth the size of the country). There is a great opportunity for Costa Rica and other ambitious countries to plan for long-term forest restoration.

If we can begin to restore a million hectares of forest by 2020, why not plan to restore a million hectares of 100-year old forest by 2120?

A trail through secondary forest at the Las Cruces Biological Station in southern Costa Rica. Photo by J. Leighton Reid.

Sometimes farmlands quickly regrow tropical forests on their own, but other times they don’t. Dr. Karen Holl, a professor at the University of California Santa Cruz, gives some rules of thumb for when we can save money on tropical forest restoration by letting nature do the work, and when we may need to invest in tree planting.

Ambitious targets are being set to restore tropical forest because of their importance in storing carbon, regulating water cycles, conserving biodiversity, and supporting the wellbeing of people who live in tropical countries. For example, the 20 × 20 Initiative aims to restore 20 million hectares of tropical forest in Latin America by 2020. This represents an area slightly smaller than the country of Ecuador. One big question is: How are we going to restore forests at this scale with limited funds?

One of the cheapest ways to restore forest is to let nature do the work and leave forests to recover on their own. This works in some sites where forests regenerate quickly. In other cases, usually sites that have been used intensively for agriculture, the land may be covered by tall grasses (up to 3 meters, or 10 feet high) for years. Our past research shows that even within a small region, the rate of natural forest recovery varies greatly.

Natural forest recovery is highly variable in southern Costa Rica, even after a decade of recovery. Left: slow recovery on a former farm, still dominated by non-native grasses, with an open canopy and little tree recruitment. Right: speedy recovery on a former farm, with virtually no grass cover, a closed canopy, and diverse tree recruitment. Photos by Andy Kulikowski.

So, how do we predict which sites will recover quickly and which ones need some help in the form of clearing pasture grasses and planting trees? If we could develop some rules of thumb it would help land managers to more efficiently allocate scarce restoration funds.

To answer this question, we drew on our long-term study on tropical forest restoration in southern Costa Rica. We have research plots at 13 different sites where we removed the land from agriculture and let the forest recover on its own. Each year we measure grass cover, tree canopy cover, and how many and what species of new tree seedling establish in the plots. We have also quantified the forest cover surrounding the plots, the nutrients in the soil, and how long cows had grazed the sites in the past.

We found that two easy-to-measure variables explained on average two-thirds of variation in forest recovery 7 years later; those were the amount of grass cover and tree canopy cover measured after only 1.5 years. Plots that had more canopy cover and lower grass cover early on had a closed tree canopy and lots of forest tree seedlings from many species after nearly a decade. We were surprised that the amount of surrounding forest cover and soil nutrients did not explain much of the variation in forest recovery.

Rules of thumb for predicting tropical forest regeneration on farmlands. Forests grow back quicker when there is not too much grass, a little bit of shade, and many tree seedlings already present. Illustrations by Michelle Pastor.

Of course, our results need to be tested in other recovering tropical forests. But, if they hold true, this is good news! It means that land owners and managers just need to wait a year or two and then measure the tree canopy and grass cover. If some trees have established and are starting to shade out the grasses, land managers can use the low cost method of leaving the site to recover naturally. If the site is mostly a monoculture of dense grass, then the site is a good candidate to plant native trees. Planting trees takes more resources since it is necessary to clear around the native tree seedlings for a couple of years until they grow taller than the grasses. At least now there are some general guidelines to help chose where to invest the extra effort.

CCSD scientist Leighton Reid and Lyon Arboretum director Rakan Zahawi write about giant fig tree cuttings: how to make them and why some grow better than others.

Choosing the right species to include in a restoration project is a hard choice, but in the economy of nature, some species earn a bigger ROI than others. For example, Pacific sea otters maintain kelp forests by eating sea urchins, and wolves in Yellowstone National Park allow aspen groves to regenerate by scaring away tree-munching elk. These vital creatures are called “keystone species” because they hold ecosystems together, much like the keystone in an arch.

Plants can be keystone species too. Around the world there are about 800 species of fig trees, and they hold tropical forests together by providing food for a wide array of animals. On any given day, the busiest tree in a rain forest is likely to be a fig tree with fruits. Monkeys, birds, bats, and others gather at fig trees to eat, and in the process, they deposit seeds of other plant species that they have been carrying in their guts. This chain of events, repeated day after day, often turns the area beneath a fig tree into a hotspot of plant diversity.

A few years ago, we had an idea to plant keystone fig trees in young forests in Costa Rica. We wanted the figs to grow as fast as they could, so instead of planting seedlings, we planted cuttings – big ones. With help from our local collaborator, Juan Abel Rosales, we cut dozens of twelve foot-long branches from eight species of fig trees. We stripped off all of their leaves to keep them from drying out, and then we planted our figs trees in shallow holes.

Rakan Zahawi (delighted!) poses with a three year-old fig stake.

To our delight, many of the fig trees grew!

The ones that did the best came from a special group, the subgenus Urostigma. Many figs in this group have a unique life strategy. They begin their lives in the top of a tree when their tiny seeds are deposited on a branch by a bird or some other animal. As they grow in the treetop, they send long roots down to the ground, and these roots harden and fuse together, forming a lattice-like trunk. Over time, these figs kill their host trees by taking most of the water, nutrients, and light. They also keep the host tree from growing outwards, giving them the nickname “strangler figs”. Maybe the ability to transform a flimsy, dangling root into a solid trunk is related to these figs being able to grow from cuttings.

To find out how well our planted fig cuttings might survive over the long-term, we also tracked down some fig cuttings that we had planted in 2004. We were happy to learn that out of the trees that survived for their first three years of life, all of them were still thriving a decade later.

Full disclosure: planting large cuttings is not a new idea. Farmers in many parts of the tropics plant trees this way to create ‘living fences’ – with all of the normal fixings like gates and barbed wire, but with a row of living trees instead of dead posts. The advantages for farmers are many – their fences don’t rot and fall apart (that happens quickly in the tropics); the trees provide shade for cattle; they have a constant source of new fence posts (by cutting off a limb); and in some cases they can feed the young shoots to livestock.

Big cuttings have big benefits for restoration too. Not only are planted trees already several feet tall, you also get to skip the pricey nursery phase, and, most excitingly, cuttings have a tendency to fruit quickly.

Some of our young fig trees are now making fruit, but we will have to wait a bit longer to see whether they start attracting more big animals and whether those animals carry more tree seeds into our young forests. For now, we can say that others who are interested in growing keystone figs for forest restoration may have the best luck by working with the stranglers.

How to grow an instant fig tree. (A) Remove a long, thin branch segment from an adult tree. The red arrow shows a cut branch. (B) Strip the cuttings of their leaves to keep them from drying out, then carefully transport cuttings so as not to damage cortical tissue. Here, cuttings are padded by a foam mattress. (C) Remove the bark from a ring on the cutting to promote root growth. Here, a ring is being cut about 20 cm (8 in) above the base so that it will be just below the soil surface when planted. (D) Dig a shallow hole and plant the cutting. Be sure that the cutting is firmly planted to prevent it from toppling, but take care not to compact the soil too much around its roots. Photos by Rakan Zahawi.

Andres Santana is the graduate program coordinator at the Organization for Tropical Studies. During a recent fieldtrip in southern Costa Rica, he and CCSD restoration ecologist Leighton Reid compared notes on using fig stakes for ecological restoration.

Tropical beaches are many things to many people. To plants, beaches are hot, sandy, and salty – complicating their restoration.

Costa Rica has 1228 km (763 mi) of coast line – including 1016 km on the Pacific side and 212 km on the Caribbean. Along Costa Rica’s northern Pacific coast, the beach forms the natural edge of the dry forest. Farther south the adjacent forest is more humid. Giant trees, 40 m or more in height, grow right up to the high tide mark, particularly along the Caribbean.

But as with so many tropical ecosystems, Costa Rica’s coastal forests have been subject to human impacts. Many shoreline forests were cleared for cattle ranching, and exotic grasses were introduced as forage. Some of these grasses are fierce competitors and prevent tree seedlings from establishing, even long after the pastures have been abandoned.

Playa Hermosa, before (left) and after (right) planting 2-m long cuttings of a coastal fig species (Ficus goldmannii).

In 2009, a small non-profit organization, Costas Verdes, was formed to restore coastal forests along degraded shorelines, particularly wildlife refuges. The restoration work was initially challenging; tree seedlings were hard to establish along the coast because of the harsh environment – high temperatures and salinity and lack of freshwater were among the most significant obstacles. Not to mention the invasive cattle forage grasses.

Coastal restoration at Playa Hermosa

Playa Hermosa, a surfing destination on the Central Pacific coast, was among the most heavily deforested project sites. This area, part of a wetland and river estuary, was declared a national wildlife refuge in 1998. By 2009, very little forest had naturally regenerated. This led Costas Verdes to implement a restoration project at this beach. Planting plots were established where invasive grass was removed. In other areas, grasses left intact, as a comparison. It quickly became evident that tree seedlings were outcompeted by the grass. Those in the cleared plots grew better, but they still faced the other coastal habitat challenges.

Some native trees are resistant to hot substrates and high salinity, but these species were not available in tree nurseries, most of which focused on ornamental species. This meant that seedlings needed to come from locally collected and germinated seeds. We realized that this would take time to get going. Tree seedlings under 50 cm rarely survive, even if they have the proper coastal adaptations.

To accelerate the restoration, we decided to use tree cuttings rather than growing seedlings from seed. A colleague suggested Ficus goldmannii as a candidate species, so in 2011 we conducted a planting trial. We planted 225 2-m long cuttings. Of these, 195 (87%) survived their first year. By the second year all 195 survivors had become established and were quickly providing canopy cover and lowering the temperature of the sand.

An established fig stake with a dense canopy. Note the weak, patchy grass below it.

Once fig stakes created some canopy cover, we brought in other tree species – mostly from the coastal tree nursery that we created. Shade from the fig canopy also began to inhibit the invasive grasses, which require high sunlight to photosynthesize efficiently. Reduced competition with these grasses allowed other tree seedling species to survive.

In this instance Ficus cuttings turned out to be useful in promoting restoration. We have since used cuttings for other plots with similar success.

Steve Roels is a PhD candidate in the Department of Integrative Biology at Michigan State University. His research asks how trophic cascades interact with tropical forest restoration. When not in Panama, he enjoys documenting biodiversity and restoring native vegetation on his own 6.2 acres of Michigan.

Tropical field biology has a lot of uncertainty built into it. The scientific community is still barely scratching the surface of tropical biodiversity and the immense complexity of biotic interactions (relationships between organisms). Biologists, myself included, often get lulled into thinking of the tropical climate as a stable abiotic backdrop that lies behind the great drama of biotic interactions. But what happens when those abiotic conditions change abruptly and dramatically?

The current El Niño event in the Pacific is now regarded by meteorologists as one of, if not the, strongest El Niño events ever recorded. The North American media understandably focuses on how El Niños affect our continent; usually wetter West Coast winters and dryer, warmer Midwest winters. What many North Americans don’t realize is that El Niño events have their most profound effects on Pacific countries in the tropics.

El Niños are one extreme of a much larger climate pattern, the Southern Oscillation. The El Niño-Southern Oscillation (ENSO) is an erratic seesaw of Pacific surface water temperatures from warm to cool (La Niña events) and back again. Temperature swings from one extreme to the other occur every few years (on average about 5) and “the switch” is often flipped very abruptly, shifting ocean currents, air pressure, and precipitation throughout the eastern Pacific. It is important to keep in mind that ENSO events are not “bad” per se, just different, and that creates biological winners and losers.

In central Panama, where I research bird communities in forest restorations, El Niño conditions generally bring warm coastal waters and drought. I say “generally” because each El Niño is like a snowflake—there are some basic patterns, but every event is unique. This El Niño is sticking to the pattern: central Panama is currently experiencing a severe drought and creating headaches for many of my colleagues at the Smithsonian Tropical Research Institute (STRI). The drought has played havoc with the frog biologists, who are waiting for mating frogs, who have, in turn, often been waiting for rain. Coral researchers are scrambling as abnormally warm waters cause coral bleaching. However, some scientists view this El Niño as an opportunity because it could be considered a proxy for future climate. The El Niño is compounding the warming effects of global climate change, putting 2015 on track to be the warmest year on record. A friend of mine who studies tree physiology and water use in forest restorations says she is getting great data. After all, a key challenge for restoration ecology is deciding what we restore to. An ecosystem that tries to match what was formerly present? Or one that will continue to thrive in an uncertain future?

The Agua Salud restoration site. The blocks of vegetation in the landscape are different experimental tree planting treatments. Lake Gatun, part of the Panama Canal, lies in the haze on the horizon. Lake levels are anticipated to drop to record lows this dry season.

The effects of current El Niño on my own research are difficult to assess. I study trophic cascades (basically, ripples in food webs) at STRI’s Agua Salud forest restoration project, especially focusing on birds, insects, and trees. I conducted an experiment this past July-August, which is normally the heart of the wet season, but was instead a historic drought. How this drought effected tree growth, insect populations, and bird behavior—all components of my study—is hard to say. Prior research on ENSO effects on trophic relationships is limited (it’s hard to plan research around an unpredictable and irregular event!) but some long-term studies have found large ENSO effects on food webs in Panama and Chile.

When I returned to the United States after my field season and talked with my research advisor about the uncertainty the El Niño brought to my study, she said, “You’re going to hate me for saying it…” I replied, “I already know what you’re going to say.” Maybe I need to do the experiment again next year.